[bullet lists]

Petmail also:

 requires no changes to the intermediate mail infrastructure: SMTP relays are
 expected to perform message routing, nothing more

 retains the capability for full anonymity, even makes it more convenient

 despite the title, does not actually seek to prevent UCE. Specifically it
 prevents non-pre-authorized machine-generated mail, but that's the only
 economical kind of spam out there anyway.